feat(bindings): Associate an application context with a Connection

[goatgoose]

Resolved issues:

Resolves #4264

Description of changes:

Adds new APIs to set and retrieve an arbitrary struct on a Connection.

This allows applications to associate connection-specific application data with each connection, and retrieve the data from within s2n-tls callbacks.

Call-outs:

I made an example that demonstrates how the application context can be used with the session resumption callback to solve #4264. I intend to submit this example in a subsequent PR, but you can view it on a branch for now if you want.

Testing:

I added new tests that set and retrieve the application context.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.